The Mpemba effect is named after a Tanzanian school student who in 1969 noticed that hot water froze faster than cold while making ice cream.
He shared his discovery with his teachers, but it was some years before the effect was recognised by scientists. Mpemba is one of many science prodigies who have made their contributions early in life. Albert Einstein is usually pictured as an elderly, grey-haired man. But it was as a vigorous young man that he made many of his greatest discoveries; and he first questioned the world when he was a child.
The genius of Einstein, and of other great scientists, is that they asked child-like questions with earth-shattering consequences. Einstein's innocent childhood question was: "What would the world look like if I could ride on a beam of light?" And the conclusion he reached was that by travelling at the speed of light, he would free himself from passing time.
He would be alone in a box of time and space - his experience would be different from that of a person standing still. The difference between them he called "relativity".
There are science prodigies like Mpemba and Einstein in our schools today.
Take, for example, Zach Bjornson-Hooper, a 13-year-old from Alamo, California. His discovery, reported in the American press last year, may not compare with the principle of relativity, but it could be a good deal more important to you.
The well-travelled, home-schooled Zach had noticed that he and his family had stomach trouble after long flights. They put it down to foreign food and jet-lag, but Zach noticed that water on the flight was served from jugs, and must have come from a tank somewhere. His family always drank bottled water when sailing their yacht, never from the tank. Zach collected samples from their next nine flights, and tested them at home. He described what he grew on sterile agar plates as "smelling like rotten peanut butter". Seven samples contained E coli, faecal coliform and other bacteria. One contained insect eggs. His mother set about advising other passengers about drinking airline water.
Zach's and Mpemba's work has been celebrated. It was very different a century and a half ago for the young Manya Slodovska - later to be named Marie Curie. She was fascinated by science from an early age. The occupying Russians forbade all "higher studies" in Poland, which led her to study in France - at the Sorbonne. Like Einstein, she asked challenging questions and sought out the answers.
The answer to Mpemba's question is complex. When cold water freezes, it first forms a layer of ice on its surface, which acts as an insulator and slows down the freezing below it. Convection currents keep warm water moving, preventing this insulation layer from forming. Warm water also evaporates faster than cold. So there is less water to freeze. And the process of evaporation is cooling, too. Finally, heating water drives out gases like oxygen and carbon dioxide. Impurities such as these lower the freezing point of water - as salt does on an icy road - and so would make cold water harder to freeze.
The British Association for the Advancement of Science, through its Young Investigators organisation, offers support to science clubs for five to 13-year-olds, activity days and a magazine. Many local authorities arrange Saturday clubs and summer schools for young scientists. And you can encourage budding scientists by inviting questions. One Nobel prize winner remembers his mother asking him not "What did you do in school today?" but rather "Did you ask a good question?"
John Stringer is a science writer and teacher trainer. The British Association for the Advancement of Science: www.britassoc.org.uk
The Mpemba effect: www.school-for-champions.com mpemba.htm